U.S. Marines with the 11th Marine Expeditionary Unit carry 155mm rounds to an M777 Howitzer gun line in preparation for fire missions in northern Syria. The sea-foam green rounds on the truck are white phosphorus munitions. (Photo courtesy Lance Cpl. Zachery Laning, U.S. Marine Corps)

The U.S.-led coalition in Iraq and Syria appears to have used white phosphorus-loaded munitions on at least two occasions in densely populated areas of Mosul and in the Islamic State’s de facto capital of Raqqa, according to videos posted online and human rights groups.

The often-controversial munitions are common in western militaries and are used primarily to create smoke screens, though they can also be dropped as an incendiary weapon. When a white phosphorus shell explodes, the chemical inside reacts with the air, creating a thick white cloud. When it comes in contact with flesh, it can maim and kill by burning to the bone.

While international humanitarian law stipulates that civilians must be protected from all military operations, it also says that countries must take even more care when using white phosphorus. Additionally, because of the weapon’s ability to cause grievous and inhumane injuries, rights groups caution against using white phosphorus to kill enemy troops if other weapons are available.

On Thursday, footage posted by the activist group Raqqa is Being Slaughtered Silently showedthe signature spread of airburst white phosphorus munitions — probably M825 series 155mm artillery rounds — exploding over eastern Raqqa, the same area where U.S.-backed Syrian fighters made advances earlier this week.

U.S. Army Col. Ryan Dillon, a spokesman for the U.S.-led coalition fighting in Iraq and Syria, would not confirm the use of the munition but said in an email that the U.S. military uses it in “accordance with the law of armed conflict” and that white phosphorus rounds are “used for screening, obscuring, and marking in a way that fully considers the possible incidental effects on civilians and civilian structures.”

“The coalition takes all reasonable precautions to minimize the risk of incidental injury to non-combatants and damage to civilian structures,” he said.

The Pentagon posted photographs of Marine M777 howitzers in Syria — deployed to support the Raqqa operation — with a pallet of white phosphorus munitions in May. The image was taken in March, and while the unit in the photograph probably has returned to the United States, its replacement is likely using similar munitions.

Mary Wareham, the advocacy director at Human Rights Watch’s arms division, said in email that the group is still trying to determine the veracity of the videos, but the munitions look similar to the ones used Saturday in the Iraqi city of Mosul. Wareham said that in Mosul it appeared that the rounds exploded close to the ground, in “an attempt to minimize the footprint of the effects.”

When M825 rounds explode, they jettison roughly 115 felt wedges that are impregnated with white phosphorus. If exploded high above the ground, the wedges can spread over a greater distance, starting fires over a wide area. In Mosul, smoke munitions were used, according to a statement by Iraqi forces, to provide cover for civilians targeted by Islamic State snipers.

While the Islamic State controls only a few remaining neighborhoods in the western part of Mosul, the small area is packed with tens of thousands of civilians, raising concerns among rights groups that the heavy fighting will kill hundreds of civilians before the fighting ends.

“White phosphorus should not be air burst over populated areas due to its indiscriminate effect but it’s not clear from available information that civilians are in the area,” Wareham said. “The [Iraqi Security Forces] is claiming that it used white phosphorus to protect civilians. As such, more information is needed to determine whether the white phosphorus use here is lawful.”

In Raqqa, however, the footage shows the munitions bursting relatively high off the ground over a cluster of buildings. It is unclear if Islamic State fighters are in the area, but thousands of civilians are known to be still in the city. In the days leading up to the battle in the city, U.S.-backed Syrian fighters called on civilians to evacuate prior to the offensive, but after commencing their attack, they’ve now told those inside to shelter in their homes and avoid Islamic State positions.

A report from the casualty-monitoring group Airwars.Org indicates that this mixed messaging has created some confusion among the civilian population in the city and that despite the fighting, some are still evacuating.

The Pentagon has admitted to killing roughly 500 civilians in the nearly three-year-old war against the Islamic State. Monitoring groups, such as Airwars, say that number is extremely conservative. The U.K.-based organization claims that roughly 3,800 civilians have been killed by the U.S.-led coalition.